In automatic test equipment (ATE) systems, a phase delay register is often used to control the placement of a clock signal or a strobe with respect to the input data signals. Sometimes the strobe signal is not available for direct measurement. The strobe position with respect to the input data waveform may be indirectly captured via placing strobes on a number of bits in a known input waveform and observing the number of errors produced, which gives a statistical indication when the strobe position coincides with the input bit transition time. In order to accurately determine the strobe positions, there is a need for time vernier calibration, or more precisely, a need for calibrating the actual delay of a strobe signal with respect to the programmed delay setting of a phase delay register.
Among the existing time vernier calibration methods, some require access to the input and output signals of the time vernier or time delay generator, or at least observability of periodic output signals from the time delay generator. For example, the time vernier can be placed in a feedback loop to make a ring oscillator so that the resulting signal frequency can be measured to determine the actual delay. Alternatively, the input and output signals from a timing generator can be observed by an oscilloscope or any other external time measurement instrument and the actual delay can be measured with the increase in the programmed time delay. However, these techniques are not applicable when the input and output signals of a time vernier or time delay generator are not accessible.
Time vernier calibration can also be accomplished through synchronizing an input signal with respect to the strobe signal and subsequently introducing known delays on the input signal. Under this approach, the strobe delay can be adjusted to find the edge transition of the delayed input signal and the known delay as compared to the programmed delay value. If the time verniers to be calibrated have pico-second step size, however, it is generally difficult and expensive to generate known delays with such sub-picosecond resolution and accuracy as necessary to calibrate the time verniers.
Another approach for time vernier calibration is to create accurate delay increments via “precession” or “walking” of two signals having closely spaced but not identical periods. The problem with this approach is the difficult alignment of two signals, namely, how to synchronize the two signals such that they can be consistently time aligned for repetitive measurements.
In light of the above, a need exists for an inexpensive and easy-to-implement method and apparatus for time vernier calibration that can greatly enhance existing ATE systems.